The charging of a battery for a battery-powered device occurs through various constant-voltage or constant-current modes depending upon the state of the battery charge. As implied by the name, the charging voltage stays constant at some fixed level during a constant-voltage mode whereas the charging current may vary. Conversely, the charging current stays constant during a constant-current mode whereas the charging voltage may vary. The proper sequencing and control of the constant-voltage and constant-current modes is vital with regard to battery longevity. For example, a smartphone's battery is often integrated and non-removable. If the battery for such a device is damaged, the entire smartphone must be replaced. It is thus conventional for a mobile device to include a battery management circuit that controls the charging voltage and charging current for the battery.
Since the battery management circuit within the mobile device is controlling the charging voltage and charging current applied to the battery, the tolerances for the switching power converter supplying power to the mobile device are relaxed. An example charging system is shown in FIG. 1. A switching power converter such as a flyback converter 100 converts an input voltage into a regulated output voltage V_out during a constant-voltage mode of operation. A battery-powered device such as a smartphone 105 includes a battery management circuit 110 that controls the constant-voltage or constant-current charging applied to a battery for powering a system 115 using the output power from flyback converter 100. For example, in a constant-voltage mode, battery management circuit 110 regulates the constant output voltage from flyback converter 100 into a constant charging voltage for the battery. Similarly, in a constant-current mode, battery management circuit 110 regulates the constant output current from flyback converter 100 into a constant charging current for the battery. This regulation by battery management circuit 110 provides some tolerance for the regulation in flyback converter 100.
For example, the output voltage and output current tolerance for flyback converter 100 may be +/−5% as shown in FIG. 2 for the constant-voltage and constant-current modes of operation. For a desired constant-voltage mode of 5 V, the 5% tolerance means that output voltage can actually range from 4.75 V to 5.25V. To maintain regulation during constant-voltage operation, a primary-side controller (not illustrated) in flyback converter 100 needs some means of sensing the output voltage. In a primary-only-feedback configuration, the output voltage may be sensed through an auxiliary winding (or through the primary winding). As also shown in FIG. 2, an analogous tolerance range occurs for the output current during the constant-current mode of operation. Just like the output voltage, a primary-side controller cannot sense the output current directly but must instead do so indirectly. To indirectly sense the output current, a primary-side controller may sense the peak current in a switching cycle through the power switch using a sense resistor. The output current can then be estimated since the output current is proportional to the peak current. In particular, this proportionality depends upon a number of factors including the transformer turns ratio, the switching period, the sense resistor resistance, and the transformer reset time.
Such indirect sensing of the output current is adequate if the output current tolerance is fairly large such as shown in FIG. 2. But portable devices have been developed in which battery management circuit 100 is either absent or bypassed in what is denoted herein as a direct-charge system. In a direct-charge system, the power converter itself is directly charging the portable device's battery. But note that modern smartphones typically have the battery non-removably integrated into the phone so that if the battery is defective, the entire smartphone becomes defective. This is especially problematic given the high cost of modern smartphones. It is thus critical that a power converter such as flyback converter 100 regulate the constant-voltage and constant-current modes with considerable precision so that the health of the mobile device's battery is preserved. The tolerance for these operating modes is thus reduces in direct-charge systems (e.g., +/−1% of the desired constant current or constant voltage). Despite this reduced tolerance, note that there is a substantial tolerance on the component parameters controlling the proportionality between the peak current and the output current such as the sense resistance and the transformer reset time. It is thus problematic for primary-side-regulation of the output current to achieve the necessary tolerance during constant-current operation.
There is thus a need in the art for flyback converters having improved primary-side regulation of the output current during constant-current operation.